1. Field of the Invention
The present invention relates to a method for producing an electrophotographic photosensitive member, more particularly to a process for producing an electrophotographic photosensitive member employing dip coating.
2. Related Background Art
Dip coating has been widely practiced in coating processes for forming a photosensitive layer or an intermediate layer in production of electrophotographic photosensitive members. This method of dip coating comprises basically a means for bringing down an object to be surface-coated into a paint in a coating bath and subsequently pulling up the object at an appropriate speed.
Such a method of dip coating is suitable for production of electrophotographic photosensitive members having a seamless drum shape or the like in comparison with other coating methods in viewpoints of productivity, and the like. This method, however, frequently causes coating irregularity and coating defects depending on characteristics of the paint employed. In particular, defects such as coating irregularity, small lumps, and streaks are known to be liable to occur when a paint containing fine pigment particles is applied in a thin layer less than 10 .mu. thick. The main causes of defects include (i) turbulence of paint in a paint coating bath, (ii) sedimentation of pigment particles in a paint coating bath, (iii) coagulation of paint by thixotropy in a paint coating bath, and other causes. In the cases where a paint containing fine pigment particles having an anisotropic shape is used, particularly the third cause, namely thixotropy, exerts harmful influences. In an extreme case, a part of the paint undergoes gelation in the piping to make paint application impracticable, which is a serious problem in production of electrophotographic photosensitive members.
To solve such problems, the measures hitherto taken include (i) treatment of the paint itself such as addition of a dispersant, pretreatment of the pigment, selection of the optimum particle size of the paint, and so on, (ii) provision of an agitation apparatus in the paint coating bath or in the piping, and like methods.
However, the former measure (i) is not necessarily effective because there are many restrictions in consideration of the characteristics of electrophotographic photosensitive members, while the latter measure (ii) is also not effective because the agitator itself gives rise to turbulence of the paint. Use of ultrasonic wave was proposed, but it was impracticable because of generation of noise and heat.